regmap: add a new macro:REGMAP_IRQ_REG_LINE(_id, _reg_bits)

if there are lots of irqs for a device and the register addresses for these
irqs is continuous, we can use this macro to initialize regmap_irq value.

Signed-off-by: Tony Xie <tony.xie@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>